Enhancement of Nitrate Reductase Activity by Benzyladenine in Agrostemma githago

Nitrate reductase activity in excised embryos of Agrostemma githago increases in response to both NO(3) (-) and cytokinins. We asked the question whether cytokinins affected nitrate reductase activity directly or through NO(3) (-), either by amplifying the effect of low endogenous NO(3) (-) levels, or by making NO(3) (-) available for induction from a metabolically inactive compartment. Nitrate reductase activity was enhanced on the average by 50% after 1 hour of benzyladenine treatment. In some experiments, the cytokinin response was detectable as early as 30 minutes after addition of benzyladenine. Nitrate reductase activity increased linearly for 4 hours and began to decay 13 hours after start of the hormone treatment. When embryos were incubated in solutions containing mixtures of NO(3) (-) and benzyladenine, additive responses were obtained. The effects of NO(3) (-) and benzyladenine were counteracted by abscisic acid. The increase in nitrate reductase activity was inhibited at lower abscisic acid concentrations in embryos which were induced with NO(3) (-), as compared to embryos treated with benzyladenine. Casein hydrolysate inhibited the development of nitrate reductase activity. The response to NO(3) (-) was more susceptible to inhibition by casein hydrolysate than the response to the hormone. When NO(3) (-) and benzyladenine were withdrawn from the medium after maximal enhancement of nitrate reductase activity, the level of the enzyme decreased rapidly. Nitrate reductase activity increasd again as a result of a second treatment with benzyladenine but not with NO(3) (-). At the time of the second exposure to benzyladenine, no NO(3) (-) was detectable in extracts of Agrostemma embryos. This is taken as evidence that cytokinins enhance nitrate reductase activity directly and not through induction by NO(3) (-).